Radio frequency identification (RFID) systems typically include at least one reader and a plurality of transponders, which are commonly termed credentials, cards, tags, or the like. Each transponder is an active or passive radio frequency communication device which is directly attached to or embedded in an article to be identified or otherwise characterized by the reader. Alternatively, the transponder is embedded in a portable substrate, such as a card, tag, or the like, carried by a person or an article to be identified or otherwise characterized by the reader.
An active transponder is powered up by its own internal power supply, such as an internal battery, which provides the operating power for the transponder circuitry. In contrast, a passive transponder is dependent on the reader for its power. The passive transponder typically consists of an integrated circuit (IC) chip coupled to a resonant LC circuit which has a capacitor and an inductive antenna in parallel or in series. The reader “excites” or powers up the passive transponder by transmitting excitation signals of a given frequency into the proximal space surrounding the reader. When the transponder resides in the proximal space, its inductive antenna receives the excitation signals which are converted into the operating power for the IC chip of the recipient transponder.
The powered up transponder generates transponder data signals which are in the form of electromagnetic waves embodying information such as identity data or other characterizing data stored in the memory of the IC chip. The transponder data signals are characterized by a specific carrier frequency which generally corresponds to the frequency of the excitation signals. The carrier frequency is inter alia a function of the transponder LC circuit and is often unique to the particular manufacturer of the transponder used to generate the transponder data signal. The transponder manufacturer establishes a desired carrier frequency of transponder data signals by tuning the LC circuit to a resonant frequency which corresponds to the desired carrier frequency.
The resonant frequency (and correspondingly the carrier frequency) of commercially-available passive transponders conventionally employed in RFID applications generally fall within either a low frequency range or a high frequency range. The low frequency range extends about a nominal low frequency of 125 kHz and is typically within a range of 100 to 150 kHz. In contrast, the high frequency range extends about a nominal high frequency of 13.56 MHz. Low frequency transponders are commonly termed proximity credentials and high frequency transponders are commonly termed smart credentials. It is apparent from the above that there can be significant variability in transponder frequencies even among different types of low frequency transponders or among different types of high frequency transponders. More specifically, a credential that is designed to operate at about 13.56 MHz may actually operate at a frequency of about 14 MHz or even 15 MHz depending upon the specifications to which the card was built.
In any case, the transponder data signals are transmitted via the transponder antenna into the proximal space surrounding the reader in which the transponder resides. The reader contains its own LC circuit having a capacitor and an inductive antenna which is tuned to essentially the same resonant frequency as the transponder LC circuit, thereby rendering the reader and transponder compatible. The reader LC circuit receives the transponder data signals and is coupled to additional reader circuitry, which enable the reader to “read” the transponder data signals (i.e., extract the data from the transponder data signals). Accordingly, contactless communication is effected between the reader and the transponder in accordance with a specific communication protocol, which is likewise often unique to the particular manufacturer of the transponder and/or reader.
The excitation signal generating and transmitting functions and the transponder data signal receiving and reading functions performed by the reader as described above define a mode of reader operation termed a “data transaction mode.” The data transaction mode further encompasses reader data signal generating and transmitting functions, wherein information stored in the reader memory or otherwise generated by the reader is communicated to the transponder. The manner in which the reader communicates information to the transponder is essentially the same or similar to the manner in which the transponder communicates information to the reader. As such, the reader data signals are characterized by essentially the same carrier frequency as the transponder data signals.